This invention relates to a method and machine for carrying out a masking operation in a digital manner, in a color control system of a picture reproducing machine such as a color scanner, a color facsimile, or the like.
In a conventional picture reproducing machine (e.g., a color scanner, a color facsimile or the like), a color control process such as masking, color correction, and so forth, has been carried out by electronically processing picture signals obtained by scanning an original picture through analog circuitry. This method has good stability, reliability, reproducibility, and so forth, compared with a photographic method.
However, in recent years, with the requirements on stability, reliability and, the reproducibility, becoming increasingly more demanding, a conventional analog operational circuit has become inadequate because integrated operational amplifiers, resistors, potentiometers, and many other components of the analog operational circuit depend on temperature and their rate of use. Thus, after a long period of use, the stability, reliability, and reproducibility of these elements deteriorates to the point where the components function is affected.
In order to overcome these problems, when an operational circuit is to be composed, the highest quality components are used and temperature compensation circuits are added. However, this results in a complicated operational circuit, and accordingly, in general, less reliability and increased cost.
Thus, in order to improve stability, reliability, and reproducibility of the color control process circuitry, a digitalized operational digital method has been proposed. In this method, the processing of signals is performed on a real time basis at high speed, and color correction is carried out by transforming, not calculating, the color space coordinates of input color separation signals R, G and B of additive primary colors (such as red, green and blue) into those of output color separation signals Y, M and C of subtractive primary colors (such as yellow, megenta and cyan).
If each of the red, green and blue range is divided, for example, into 2.sup.8 tone steps, or if each color is coded by eight bits, a capacity corresponding to 2.sup.24 steps for each combination of three colors is required, and consequently such a coordinate transformation method requires a memory having a large capacity. This means high cost, and thus is not practicable.
In this prior this method, the coordinate transformation is performed by a three-dimensional memory table wherein the combinations of three color recording digital signals Y, M and C are stored and are read out of the table when addressed by various combinations of three color picture digital signals R, G and B corresponding thereto. This method offers the advantage of high speed, but, in practice, a prohibitively large memory capacity is required.
Therefore, in order to reduce the required capacity of the memory, a linear interpolation method has been proposed. One proposed interpolation method divides each of the red, green and blue range into three-dimensional tone steps. The intermediate value between adjacent steps interpolated from the adjacent steps. The adjacent values coorespond to each of the possible combinations of the recording signals Y, M and C. Signals of combinations Y, M and C which are read out of the memory by the combinations of the picture signals R, G and B corresponding thereto.
However, the relation between the picture and the recording signal is better represented by a quadratic equation. Accordingly, the errors in the values derived by linear interpolation and are often beyond an acceptable limit range. In order to perform a faithful interpolation operation, a time consuming, a complex interpolation method. Thus, it is virtually impossible to carry out this complex interpolation method in a real time process.
The essential conditions for color correction, when the picture reproducing machine such as a color scanner is operated, can be itemized in the following.
1. Color correction conditions can be readily set up; PA0 2. Color correction conditions are minimized in number; PA0 3. Each color correction condition can be independently set up; PA0 4. Color correction conditions can be expressed in a simple manner for easy comparison with their standard values; PA0 5. Color correction conditions can be specified without ambiguity; PA0 6. Color correction conditions remain stable for a long period of time; PA0 7. The same results are obtained from the color correction conditions over a long period of time; PA0 8. Color correction conditions can be readily recorded in a recording medium such as a tape or a card; and PA0 9. The color correction conditions data which has been obtained experientially by operating the color scanner can be utilized for subsequent operations.
Because of the above indicated problems with analog circuitry, it is desirable to satisfy these items by a digital circuitry, it is desirable to satisfy these items by a digital method, but no workable conventional digital method was known.
For instance, in a conventional digital method utilizing the three-dimensional memory table, each color correction condition cannot be determined independently since it is closely related to other color correction conditions. Therefore, if a part of the conditions is replaced, all of the table must be changed. This means a large number of tables are required for covering all the possibilities of color correction conditions. As a result, a certain interpolation technique becomes necessary for proper color correction along with complicated conditions for carrying it out.
This conventional digital processing is essentially a coordinate transformation from a combination of three color picture signals R, G and B to a corresponding combination of three color recording signals Y, M and C by a table in a color space, as described above, and color correction conditions such as hue, saturation, brightness, color balance, and so forth, are related closely to one another. Therefore, it is difficult to indicate the magnitude of correction for these color correction conditions in the same manner as a conventional analog method.
From above description, it is understood that the conventional digital color scanners cannot satisfy all the above items, in particular, items 1, 3, 4, 5 and 9.
As regards the item 9, in particular, it is not an essential function of a color scanner, but is an important condition if the scanner is to be an efficient device.
Further, in the conventional three-dimensional coordinate transformation process, if each picture signal R, G and B is coded by a binary code having 8 bits, each combination of three picture signals R, G and B corresponds to a binary code having 24 bits, which means that the system storage capacity must be at least 2.sup.24.
The color corresponding to each combination of three picture signals R, G and B can be represented by brightness, saturation and hue, as is well-known. Brightness requires the maximum resolving power. In conventional digital methods brightness, saturation and hue are expressed by binary codes, each having 8 bits. However, brightness and saturation share a common component, i.e., and equivalent gray density component, and hence saturation possesses a redundancy. Furthermore, it is well known that the resolving power for hue may be reduced as compared with that for brightness, without impairing the color of reproduced pictures.
Accordingly, in processing, color data the sampling steps for saturation and hue may be compressed, and thus these two factors may be represented by binary codes having 6 bits (i.e. altogether 2.sup.20 is all that is necessary for each combination of three color picture signals R, G and B).